Sound reproducing system



Feb. 20, 1962 J. BOBB ET AL SOUND REPRODUCING SYSTEM Filed Oct. 29, 1956 United States Patent C) SOUND REPRODUCING SYSTEM Lloyd J. Bobb, Glenside, Robert B. Goldman, Philadelphia, and George C. Maling, Jr., Glenolden, Pa., as- Y signors, by mesne assignments, to Philco Corporation,

Philadelphia, Pa., a corporation of Delaware Filed Oct. 29, 1956, Ser. No. 618,791

Claims. (Cl. 179-1) The present invention relates to sound reproducing systems and more particularly to an improvement in electrodynamic speakers and in sound reproducing systems employing electrodynamic speakers. Conventional electrodynamic speakers comprise a cone of semi-rigid material which is secured at its outer edge to a rigid frame. In many instances the cone is corrugated near the outer edge to provide a degree of flexibility in this region of the cone thereby to permit the cone to move as a piston under the influence of a voice coil which is secured to the cone at its apex. The voice coil is located within a cylindrical gap of a magnet which provides a substantially constant magnetic field across the gap. The magnetic field set up by the audio frequency currents within the voice coil interacts with the constant iield of the magnet with the result that physical forces are applied to the voice coil which tend to move the voice coil and the attached cone in a direction parallel to the axis of the cone. To simplify the description which follows, the direction from the apex toward the base will be considered the forward direction of the cone and the speaker. Movement of the cone so that the apex approaches Ithe normal plane of the base will be spoken of as movement of the cone in the forward direction. Movement of the cone in the opposite direction will be spoken of as movement in the backward direction.

In the past it has been the practice to center the voice coil longitudinally within the gap so as to avoid or at least minimize the effect of the fringing magnetic elds at the two ends of the gap. Ideally, an electrodynamic speaker of the type described above will produce an acoustic wave in the surrounding medium which is the exact counterpart of the corresponding electrical signal supplied to the voice coil. As a practical matter an electrodynamic speaker generates harmonics of the component frequencies to be reproduced which distort the acoustic Wave produced by the loudspeaker. There are several unavoidable sources of even harmonic distort-ion in electrodynamic speakers. The cone acts as a more eilicien-t piston to move the air in front of it when it is moving in the forward direction than it does when it is moving lin the opposite or backward direction. Also the nature of the cone suspension at its base or outer edge is such that it is physically easier to move the cone in the forward direction than in the opposite direction. An-

,other source of second harmonic distortion is found in the voice coil-magnet assembly. It is economically unfeasible to employ magnets of suicient size to insure that the voice coil will not move into a region of fringing field on the relatively large excursions of the cone which take place at low frequencies. Therefore for large excursions of the voice coil the forces imparted thereto will not be a linear function of the current 'owing inthe voice coil as it is displaced from its rest position. The asymmetry of the Ifling-ing iield is such that -the forces will be greater for displacement in one direction, usually the backward direction, than they will for equal displacements in the opposite direction. All of these inherent limitations of the electrodynamic speaker cause the positive half cycles of the electrical signals applied thereto to be reproduced differently than the negative half cycles. It isY well known that this type of distortion introduces even harmonic components of the signals to be reproduced. It

3,022,377 Patented Feb. 20, 1962 ICC tor-tion by employing cones and magnets of special design or otherwise changing the basic construction of an electrodynamic speaker. These attempts have not been altogether succeslsful and they add greatly to the cost of manufacturing an electrodynamic speaker. As mentioned above, a-t least some of the distortion could be eliminated by increasing the axial dimension of the magnet pole pieces, thereby to lengthen the axial dimension of the gap, but this requires a substantial increase in the amount of magnetic material employed. It is known that harmonic distort-ion at low frequency can be reduced by employing lar-ger cone diameters. However, a 15" speaker having the same power output as two 10" speakers may require 27 oz. of magnetic material as compared to a total 131/2 oz. for the two l0 speakers of similar design. A further disadvantage of employing cones of larger diameter is that cones having a diameter of 10" will remain piston to approximately 1000 cycles wh-ile 15 cones start to break up at -approximately 500 cycles. Also, two 10I speakers mounted one above the other are less directional in a horizontal plane than a single 15" speaker. The only major disadvantage of using two 10 speakers in place of a single 15" speaker is the increased harmonic distortion which normally results at low frequency. A typical 15" speaker operated with 8 watts Iinput may have considerably less than 10% second harmonic distortion at S0 cycles. Two l0" speakers of similar design may have second harmonic distortion in excess of 20% under similar conditions.

Therefore. it is an object of the present invent-ion to provide a novel speaker construction which produces an increase in the effective dynamic amplitude range of the speaker when included in a multiple speaker system.

It is a further object of the invention to provide a novel speaker system employing two or more relatively small speakers which are so arranged that unwanted harmonic signals introduced by one speaker are cancelled or eliminated by harmonic signals from one or more other speakers.

Another object of the invention is to provide a speaker system employing two or more speakers which is linear over a greater dynamic range than any of the individual speakers. f

These and other objects of the present invention are achieved by mounting two or more speakers on a single baille, the spacing between the speakers being such that the speakers are acoustically coupled at low frequencies. The speakers are so constructed that the fundamental components of the signals to be reproduced are in phase for both speakers but so that the even harmonic distortion introduced by the two speakers are out of phase. In general this desired phase relationship between the fundamental components of the signals to be reproduced and the second harmonic components of these signals may be achieved either by reversing the direction of one of, the loudspeakers relative to the other, so thatA 3 signals generated by the speakers are 180 out of phase.

For a better understanding of the invention, together with other and further objects thereof, reference should now be made to the following detailed description which is to be read in conjunction with the accompanying drawings in which:

FIG; lv is a schematic drawing, partially in section, of a two speaker system constructed in accordance with the present invention;

FIG. 1A is a front elevational view of the loudspeaker system shown in FIG. 1;

FIG. 2 isv a schematic drawing showing the electrical connections of the two speakers of FIG. 1;

v FIG. 3 is a series of curves illustrating the characteristics of the speakers of FIG. l;

FIG. 4 is a series of diagrams illustrating the iield distribution of a magnet of the type shown in FIG. 1;

FIG. 5 is a schematic drawing partially in section of a second preferred embodiment of the present invention; and

FIG. 6 is a schematic drawing showing the electrical connection of the speakers of FIG. 5.

In the system of FIG. 1 two electrodynamic speakers 10 and 12 are mounted behind openings 14 and 16 in baille member 18. Baifle member 18 may be the front wall of a suitable speaker enclosure and is preferably made of plywood or similar rigid material. Speaker 10 comprises a cone 20, an annular magnet 22, a cylindrical voice coil 24, a voice coil support 26 and a support 28 for magnet 22. The base of cone 20 is secured to support 28. Speaker 10 may be mounted on baille member 18 by means of screws 36 passing through support member 28. Magnet 22 is preferably in the form of a ligure of revolution having the cross-section shown in FIG. l. If the magnet is not a complete figure of revolution it should provide at least a substantially cylindrical'gap 32 between outer pole piece 34 and inner pole piece 35 to receive the voice coil 24. Voice coil support 26 is a non-magnetic sleeve which mechanically supports the coiled conductor which forms the voice coil 24. A vsuitable support member or spider (not shown in FIG. 1) may be provided for maintaining support 26 in a coaxial relationship to the pole pieces of the magnet 22.

The speaker shown in FIG. 1 departs from the construction of conventional electrodynamic speakers only in the longitudinal positioning of the voice coil 24 within the gap 32 of magnet 22. In a conventional electrodynamic speaker, the coil 24 is normally centered within the gap, extending approximately equal distances to the left and to the right of the central plane 34a of the pole piece 34 of magnet 22. In the speaker 10 of FIG. 1, however, voice coil 24 is purposely positioned coaxially of the gap 32 but so that it is offset longitudinally in the backward direction within gap 32, extending more to the left of plane 34a than to the right of plane 34a when the voice coil 24 is in its rest position. The speaker 12, shown in FIG. l, is preferably substantially identical to the speaker 10 except for the positioning of the voice coil 42 within the lgap 44 of magnet 46. Again voice coil 42 is attached to cone 43 and is positioned coaxially of the cylindrical gap 44. However, voice coil 42 is not centered within this cylindrical gap but is offset in the forward direction so that it extends more to the right of central 'plane 48a of pole piece 48 than to the left of this planev when the voice coil is in its rest position.

Assh'own inFIG. 1A the two speakers 10 and 12 preferably'are mounted one above the other on baffle 18 in orderto provide a relatively broad distribution of acousticen'ergy 'in the horizontal plane. However, in certain applications it may be preferable to mount the speakers side by side at the same horizontal level in order to provide a narrower distribution of acoustic energy in the horizontal plane. The distance D between the centers of the two speakers 10 and 12 is made small compared to the wavelength of the lowest frequency signal to be reproduced by the speaker system of FIG. 1. This close juxtaposition of the speakers provides a relatively high degree of acoustic coupling between them at very low frequencies. For signals of higher frequency the spacing between the two speakers is a greater fraction of the shorter wavelength of the higher frequency signals. Therefore the acoustic coupling between the two speakers is less for the higher frequency components of the signal to be reproduced than it is for the lower frequency components. However, there is less harmonic distortion at the higher frequencies so less coupling is required.

FIG. 2 is va schematic diagram of the electrical connection of the two speakers of FIG. 1. In FIG. 2 an audio amplifier S0 represents the source of the signals to be reproduced. The output of amplifier is connected to the primary of a conventional matching transformer 52. The secondary of matching transformer 52 is connected to the two voice coils 24 and 42 which are connected in series. The dots adjacent voice coils 24 and 42 in FIG. 2 indicate their polarities and they are energized in phase from transformer 52.

The operation of the system of FIG. l will now be explained with reference to the characteristic curves shown in FIG. 3 and the diagrams of FIG. 4. The speakers lil and 12 operate as conventional electrodynamic speakers for signals of small amplitude. That is, even though the voice coils Iare offset, the fields in which they are located are still nearly constant for small displacements on either side of the rest position of the coils. However, owing to the oifset positioning of the voice coils, on large amplitude signals the voice coil will move into a region of lesser magnetic iield sooner on onehalf cycle of the applied wave than it will on the opposite half cycle. This will introduce an asymmetry in the acoustic wave which gives rise to signals at even harmonics of the frequencies of the various components of the signal to be reproduced. However, as will be shown presently, the effect of these unwanted harmonic components is entirely eliminated by the novel speaker system of FIG. 1.

FIG. 4 illustrates the i'ield distribution around pole pieces 34 and 35 shown at A. of FIG. 4. As Shown in diagram B of FIG. 4, the field is relatively constant in the region 6@ within the gap 32 but falls oif relatively slowly in the region 62 to the left of the gap 32 and falls oif more rapidly in the region 64 to the right of the gap 32. The reason for this is that the fringing path length through the air is longer to the right of gap 32 than it is to the left. On the left hand side of gap 32 the ilux may ilow from the outer pole piece 34 to the center pole piece 3S along the paths 64, whereas to the right of gap 32 it must follow the longer paths 66. With reference to FIG. 4 it should be noted that a voice coil centered at point 68 can move only to point 70 before it encounters a certain decrease in the field strength. However, a voice coil centered at point 72 mayl be displaced the entire distance to point 70 before encountering the same variation in field strength. Therefore the displacement will be approximately proportional to current through the voice coil over the entire region from point 70 to point 72.

In drawing the curves of FIG. 3 the assumption is made that motion of the apex of the cone towards the base, i.e. motion in the forward direction, is motion in a positive direction and that current in an arbitrarily assumed positive direction through coil 24 causes a displacement of the cone in the forward direction, that is from left to right in FIG. 1. y

Movement of ycoil 24 in the positive direction from its rest position will bring it to a region more nearly centered within the circular gap 32. Therefore it will remain in a region of strong magnetic field. For this reason there will be a relatively large cone displacement per unit of current in the voice coil. Current in a negative direction through the voice coil 24 tends to move voice coil 24 in a negative direction, that is from right to left in FIG. l. Movement in this direction displaces coil 24 further from the plane 34a and further into the region of the fringing field of the magnet 22.

As mentioned earlier it is more difficult to move the cone in the backward or negative direction, that is to pull the apex away from the base, than it is to move the cone in the positive direction. For this reason, and due to the offset positioning of the voice coil described above, the negative displacements of the cone 20 per unit of negative current within the voice coil 24 are substantially less than the positive displacements of the cone resulting from equal positive currents through the voice coil 24. This condition is illustrated in curve A of FIG. 3.

In speaker 12 the voice coil is normally offset in the forward direction from the central plane 48a of pole piece 48. As a result, movement of voice coil 42 in the forward or positive direction will place it in a region of relatively weak field from magnet 46. On the other hand, movements of voice coil 42 in the negative direction will place the coil directly within the gap in a region of strong magnetic field. Therefore relatively large positive currents through voice coil 42 will provide smaller displacements of this coil and the associated cone of speaker 12 than are produced by negative currents of corresponding amplitude. This condition is illustrated by characteristic curve B of FIG. 3.

A comparison of curves B and C will show that positive currents produce positive displacement of cones 20 and 43 in speakers 10 and 12. Therefore the fundamental components of the signals reproduced by speakers and 12 will be in phase. However, since curve A is flattened in the negative region while curve B is flattened in the positive region, the even harmonic components generated by the two speakers will be 180 out of phase. If the speakers are close enough together so that they are acoustically coupled, the even harmonic components generated by one speaker will be cancelled by the corresponding even harmonic components generated by the other speaker. That is, the over-all characteristic of the speaker system will be roughly the sum of the individual characteristics of the two speakers. Curve C of FIG. 3 illustrates that the sum of the two characteristic curves A and B of speakers 10 and 12, respectively, is approximately a straight linethat is that the displacement of the cone is proportional to the current through the voice coil over a relatively wide range of positive and negative currents. Viewed in another way, the offsetting of the voice coil has the effect of lengthening the axial distance over which the operation of the speaker is linear without the necessity of physically increasing the dimension of the gap or enlarging the size of the cone. This amounts to a considerable saving in magnetic material which is the most expensive part of the speaker. The system shown in FIG. l relies on a combination of two inherently nonlinear elements to produce an extended linear range but it should not be confused with the usual pushpull connection of elements employed to reduce even harmonic distortion. Strictly speaking the speakers 10 and 12 are not connected in push-pull for the fundamental components of the signals to be reproduced since both cones move in the same direction in response to the same applied signal.

The proper amount of offset for the voice coils of the two speakers is best determined empirically since it depends to a certain extent on the stiffness and physical configuration of the cone, the actual distribution of the magnetic field on either side of the gap and on other factors affecting the movement of the cone and the efficiency of the cone in creating an acoustic wave. Optimum Operation of the system may' require that the voice coils be offset by unequal amounts although equal offsets gave highly satisfactory results in one experimental arrangement. The fact that field fringes more rapidly in the forward direction than it does in the backward direction would tend to reduce the amount of forward offset required for voice coil 42 of speaker 12 but this factor is at least partially compensated for by the fact that cone 43 is moved more easily in the forward direction than in the backward direction, thereby requiring a proportionately greater offset in the forward direction. It is not always desirable to provide an exactly linear displacement vs. current curve for the reason that the cone is a less ecient piston when it is moved in the forward direction than when moving in the backward direction. For this reason it may be desirable to make the slope of the negative half of the displacement vs. current curve slightly greater than the slope of the positive half of the curve.

In one experimental arrangement two 10 speakers were used in which the voice coil had an axial dimension of 3%, and the outside pole pieces of the magnet had a dimension parallel to the axis of the voice coil of 1A. In one speaker the voice coil projected 1A to the rear of the pole piece, and in vthe other speaker the voice coil extended ls in front of the pole piece. This offset arrangement of the voice coils does not require any structural alteration of the speaker and does not add to the oost of manufacture. This design was found to have lower even harmonic distortion at an 8 watt, 50 cycle input Elevel than ya single l5 speaker of similar design.

FIG. 5 illus-trates a second embodiment of the present invention in which two speakers, having voice coils offset in the same direction, are employed. In FIG. 5 the two speakers and 82 are mounted one `above the other on a bale 84 having openings 86 and 88 formed therein to receive the speakers. Batlle 84 may form the front w-all of a suitable cabinet 90. Speaker 80 is mounted with the back of rthe cone facing in the direction of the listener, i.e. with the black facing to the right in FIG. 5. Speaker 82 is mounted in the more conventional manner with the front of the cone facing the listener. A suitable grille cloth 92 may be provided for concealing the speakers from the listeners. The interior of cabinet may be covered with suitable sound absorbing material.

As shown 4in FIG. 6, the voice coils of speakers 80 and 82 are connected in series across the secondary of a matching transformer 94. The voice coils are so poled that, when the cone of one speaker is moving in the forward direction with respect to baille 80, the cone of the other speaker is movin-g in the backward direction. Since speaker 80 is mounted in a reversed position, the cones of both speakers will move in the same direction in space in response to a given current. Both speakers shown in FIG. 5 have voice coils displaced in the forward direction and hence both have lthe characteristic curve shown at B in FIG. 3. However, the reversal of the physical position of speaker 80 with respect to speaker 82, and the opposite poling of the voice coils of the two speakers, has the effect of reversing the characteristic of speaker 80 about both the X and Y axes of FIG. 3 so that it is effectively as shown at A in FIG. 3. Since the operation of 4two speakers having the characteristics shown at A and B of FIG. 3 has been explained in detail, no further detailed explanation of the system of FIG. 5 is needed. While in theory `the efficiency of speaker 80 is reduced by the fact that .the back of the cone faces the listener' and the acoustical path is partially obstructed by the magnet of this speaker, tin actual practice it has been found that the system shown in FIG. 5 operates almost equally as well -as the system of FIG. 1. However, the system of FIG. 5 does have the disadvantage that it requires either a greater cabinet space than the system of FIG. 1 or an offset construction of battle 84 owing to the reversed position of speaker 80.

The system shown in FIG. will work equally well with bo-th voice lcoils displaced in the backward direction. The system of FG. 5 will also operate satisfactorily using conventional speakers which normally 4have voice coils which are centered within the gap of the magnet. The reason for this is that the physical limitations of the cone structure and its suspension ltend to cause the harmonic distortion components generated by conventional electrodynamic speakers to be fixed in phase with respect to the fundamental components of the acoustic wave. Therefore the reversal of one speaker will place the even harmonic components in phase opposition in the space directly in front of the speaker. The one disadvantage in using conventional speakers in the system shown in FIG. 5 is that the linear portion of the dynamic amplitude range is less for the conventional speakers lthan it is for speakers having ofset voice coils.

The systems shown in FIGS. l and 5 employ only two speakers. It is obvious that other speakers may be added in pairs without altering the nature of the reproduction except to increase the power and possibly change the drectivity pattern of the system. it is possible, though not desirable, to have an odd number of speakers of one orientation or construction and even number of the opposite type. In such a system the distortion introduced by one speaker would be compensated for by the signals from more than one other speaker.

In FIGS. 2 and 6 the voice coils are shown connected in series. However, a parallel connection of the voice coils may be used also.

While the invention has been described with reference to the preferred embodiments thereof, it will be apparent that various modifications and other embodiments thereof will occur to those skilled in the art within the scope of the invention. Accordingly we desire the scope of our invention to be limited only by the appended claims.

What is claimed is:

`l. An electroacoustical transducer system comprising first and second elcctrodynamic loudspeakers; each of said loudspeakers having a cone, a magnet formed with a cylindrical gap for receiving a voice coil, and a voice coil coupled to said cone and disposed at least partially within said gap with the longitudinal center of said voice coil odset from the longitudinal center of said gap; means positioninv said loudspeakers closely adjacent one another with the axis of said cone of said first loudspeaker substantially parallel to the axis of said cone of said second loudspeaker and displaced from said aXis of said cone of said second loudspeaker by an amount such that the projections of said cones of said first and second loudspeakers parallel to said axes on a plane perpendicular to said axes are non-overlapping; means connected to said voice coils of said first and second loudspeakers for supplying said first and second loudspeakers with an oscillatory electrical signal to be reproduced, the connection of said last mentioned means to said voice coils having a polarity such that the displacement in space of said cone of said first loudspeaker is in the same direction as the displacement in space of said cone of said second loudspeaker `for corresponding half cycles of said oscillatory signals supplied to said two voice coils, the forward directions of said two cones having an orientation such that the acoustical even harmonic distortion components generated by said first loudspeaker in response to the application of said oscillatory signal are in phase opposition at points in space which are equidistant from said two axes to the corresponding acoustical even harmonic distortion components generated by said second loudspeaker.

2. An electroacoustical transducer system comprising a baffle formed with first and second openings therein for receiving loudspeakers, first and second electrodynamic loudspeakers; each of said loudspeakers including a magnet comprising inner and outer pole pieces defining a cylindrical gap for receiving a voice coil, each of said loudspeakers further including a cone, and a voice coil which is coupled to said cone, positioned in said cylindrical gap, and offset from the longitudinal center of said cylindrical gap in a selected direction relative to the forward direction of the associated cone, said selected dire tion being the same for the two loudspeakers; means supporting said first and second loudspeakers closely adjacent one another and adjacent said baffle, said supporting means positioning said first and second loudspeakers so that said rst and second loudspeakers overlie said first and second openings, respectively, with the axis of the cone of said first loudspeaker parallel to and displaced from the axis of the cone of said second loudspeaker, said supporting means further positioning said first and second loudspeakers so that the forward direction of the cone of said first loudspeaker is opposite to that of the forward direction of the cone of said second loudspeaker; means connected to said voice coils of said first and second loudspca ers for supplying first land second loudspeakers with substantially identical oscillatory electrical sign-als to be reproduced, the connection of said last-mentioned means to said voice coils having a polarity such that said cone of said first loudspeaker is displaced in the forward direction and the cone of said second loudspeaker is displaced inthe backward direction for corresponding half cycles or said oscillatory signals supplied to said two voice coils.

3. An electroacoustical transducer system comprising a substantially fiat bale formed with first and second openings therein for receiving loudspeakers, first and second electrodynamic loudspeakers; each of said loudspeakers including a magnet comprising inner and outer pole pieces defining a substantially cylindrical gap for receiving a voice coil, each of said loudspeakers further including a cone, and -a voice coil mechanically coupled to said cone and positioned in said gap in offset relationship to the longitudinal center of said gap; the voice coil of said first loudspeaker being offset in a forward direction rom the longitudinal center of said gap of the magnet associated therewith, the voice coil of said second loudspeaker being offsct in a backward direction with respect to the longitudinal center of the gap of the magnet associated therewith; means supporting said first and second loudspeakers adjacent said baflie in positions such that said first and second loudspeakers are located on the same side of said baiie, such that said first and second loudspeakers ove-rlie said first and second openings, respectively, such that the forward direction of the cone of said first speaker corresponds to the forward direction of the cone of said second Speaker, and such that said loudspeakers are closely adjacent one another with the axis of the cone of said first loudspeaker substantially perpendicular to said baffle and parallel to and displaced from the axis of the cone of said second loudspeaker; means connected to said voice coils of said first and second loudspeakers for supplying said first and second loudspeakers with an oscillatory electrical signal to `be reproduced, the connection of said last-mentioned means to said voice coils having a polarity such that the displacement in space of said cone of said first loudspeaker is in the same direction as the displacement in space of said cone of said second loudspeaker for corresponding half cycles of said oscillatory signals supplied to said two voice coils, the amount of offset of said two voice coils being selected to equalize the `acoustical even harmonic distortion components generated by said two loudspeakers in response to the same signal.

4. An electroacoustical transducer system comprising a bafiie formed with first and second openings therein for receiving loudspeakers, first and second electrodynamic loudspeakers; each of said loudspeakers including a magnet comprising inner and outer pole pieces defining a substantially cylindrical gap for receiving a voice coil, each of said loudspeakers further including a cone, and a voice coil attached to said cone,`and positioned in said gap in ofset relationship to the longitudinal center of said cylindrical gap in which it is located; the offsets of said voice coils of said two speakers being in opposite directions relative to the forward directions of the associated cones, means supporting said first and second loudspeakers adjacent to and on the same side of said baflie, said supporting means positioning said first and second loudspeakers to overiie said tirst and second openings, respectively, with the axis of the cone of said first loudspeaker substantially parallel to the axis of the cone of said second loudspeaker; and means connected to said voice coils of said first and second loudspeakers for supplying said first and second loudspeakers with substantially identical oscillatory electrical signals to be reproduced.

5. An electroacoustical transducer system comprising first and second electrodynarnic speakers; each of said speakers including a cone, a magnet formed with a cylindrical gap for receiving a voice coil, and a voice coil secured to said cone and disposed at least partially within said gap; the longitudinal center of said voice coil for each of said speakers being ofset from the longitudinal center of said gap within which it is located, means supporting said speakers closely adjacent one another with the axis of the cone of said first speaker substantially parallel to the axis of the cone of said second speaker and displaced from the axis of the cone of said second speaker by a distance such that the projections of said cones on a first plane perpendicular to said axes are non-overlapping, the offset of the voice coil of said first speaker from the tudinal center of its associated magnet gap being in a dithe direction of a second plane in space parallel to said first plane and remote from said two speakers, the offset of the voice coil of said second speaker from the longitudinal center of its associated magnet cap being in a direction away from said second plane, means connecting said voice coils in circuit to first and second input terminals to which a source of oscillatory electrical signals to be reproduced may be connected, said speakers having an orientation such that a signal of given polarity impressed between said first and second terminals displaces said cones of said first land second speakers in the same direction with respect to said second plane.

6. An electroacoustical transducer system comprising first and second electrodynarnic speakers; each of said speakers having a cone, a magnet formed with a cylindrical gap for receiving a voice coil, a voice coil secured to said cone and disposed at least partially within said gap with the average operating position of the longitudinal center of said voice coil offset from the longitudinal center of said cylindrical gap, means supporting said speakers closely adjacent one another with the axis of the cone of said first speaker parallel to and displaced from the axis of said cone of said second speaker, with the projection of said cones parallel to said axes on a first plane perpendicular to said axes arranged in a substantially nonoverlapping relationship, the offset of the voice coil of said first speaker from the longitudinal center of the magnetic gap with which it is associated being in a direction of a second plane in space remote from said two speakers and parallel to said first plane, the offset of the voice coil of said second speaker from the longitudinal center of the gap of its associated magnet being in a direction away from said second plane, means connecting said voice coils in circuit to first and second input terminals to which a source of oscillatory electrical signals to be reproduced may be connected, said speakers having an orientation such that a signal of given polarity impressed between said first and second terminals displaces said cones of said first and second speakers in the sarne direction with respect to said second plane.

7. An electroacoustical system according to claim 1 wherein said loudspeakers are so oriented that the forward direction of the cone of said first speaker corresponds to the forward direction of the cone of said second speaker, and wherein said two voice coils are offset in opposite directions.

8. An electroacoustical system according to claim 1 wherein said loudspeakers are so oriented that the forward direction of the cone of said first speaker corresponds to the backward direction of the cone of said second speaker, and wherein said two voice coils are offset in the same direction.

9. An electroacoustical transducer system as in claim 5 wherein the forward direction of said cone of said first speaker corresponds to the forward direction of said cone of said second speaker, and wherein said voice coils are olset in the opposite direction with. respect to their associated cones.

v10. An electroacoustical system according to claim 5 wherein said loudspeakers are oriented with the forward direction of said cone of said first speaker corresponding to the backward direction of Said cone of said second speaker, and wherein said voice coils are offset in the same direction relative to their associated cones.

References Cited in the file of this patent UNITED STATES PATENTS 1,866,831 Wolff et a1 July 12, 1932 2,002,919 Miessner May 28, 1935 2,059,929 Bobb Nov. 3, 1936 2,632,055 Parker Mar. 17, 1953 2,727,949 Lokkesmoe Dec. 20, 1955 2,781,461 Booth et al. Feb. 12, 1957 2,789,161 Brennan Nov. 16, 1957 FOREIGN PATENTS 525,154 Great Britain Aug. 22, 1940 1,127,985 France Aug. 20, 1956 

